1. Field of the Invention
The present invention relates to communication signal mixing and filtering systems and methods utilizing an encapsulated micro electro-mechanical system (MEMS) device. Furthermore, the invention is also directed to a method of fabricating a simple, unitarily constructed micro electro-mechanical system (MEMS) device which combines the steps of signal mixing and filtering, and which is smaller, less expensive and more reliable in construction and operation than existing devices currently employed in the technology.
Micro electro-mechanical system (MEMS) technology has been proposed for the fabrication of narrow band-pass filters (high-Q filters) for various communication circuits at frequencies below 200 MHz. Ordinarily, these filters employ the natural vibrational frequency of micro-resonators in order to be able to transmit signals at very precise frequencies, while concurrently attenuating signals and noise encountered at other frequencies.
In essence, communication carrier signals at radio frequencies (RF) are normally converted to intermediate frequencies (IF) for processing such as channel selection, signal isolation and the like. This particular conversion is generally implemented by mixing a carrier signal with the sinusoidal output of an oscillator in a non-linear device so that an output signal is generated which is either the sum of or the difference between the two input signals. A band-pass filter is then employed in order to select the desired converted intermediate frequency (IF) carrier signal for processing. Thereafter, a second conversion may be implemented in order to remove the intermediate frequency carrier and extract the final communication information; for instance, such as an audio message. The same two conversion steps may also be implemented in transmission in a reverse order; in effect, proceeding from an audio signal to the intermediate frequency (IF) carrier and then to the final communication radio frequency (RF) carrier frequency.
Basically, super-heterodyne communication transceivers depend upon precision electrical filtering and carrier signal mixing for signal processing at convenient intermediate frequencies. Generally these circuits possess three stages of operation. In a first stage, the radio frequency (RF) input signals are isolated utilizing a band-pass filter and then amplified. In a second stage, this signal is then beat against an intermediate frequency oscillator signal in order to reduce (or alternatively increase) its frequency for signal processing. In a third stage, after processing, the signal may then be further modulated with another oscillator signal in order to obtain audible frequencies for communication. These same stages may also occur in a reverse sequence in translating from audio frequencies to (RF) carrier transmission.
The present invention uniquely utilizes the above-mentioned first and second stages of operation, in which the carrier frequency is changed in order to perform various functions on the signal.
The conversion and filtering steps referred to hereinabove, can be implemented through the intermediary of pure electronic circuits; however, the resultant intermediate carrier is normally considered too broad in its frequency range for precision processing. Currently, the radio frequency (RF) filter is made with the excitation of an external crystal, commonly in a transmission mode. The intermediate frequency (IF) is filtration commonly attained with the use of external surface acoustic-wave (SAW) filters. The use of these two components which are ordinarily provided externally of the integrated circuit which is employed for signal amplification and processing, increases system complexity and adds to fabrication costs.
2. Discussion of the Prior Art
Typically, MEMS resonator filter devices are fabricated through the intermediary of standard integrated circuit masking/depositions/etching processes. For instance, specific details regarding the manufacture and structure of MEMS band-pass filters are readily disclosed in the following publications:
1) "Micromachined Devices for Wireless Communications", C. T.-C. Nguyen, L. P. B. Katechi and G. M. Rebeiz, Proc. IEEE, 86, 1756-1768. PA0 2) "Surface Micromachining for Microelectromechanical Systems", J. M. Bustillo, R. T. Howe and R. S. Muller, Proc. IEEE, 86,1552-1574 (1998). PA0 3) "High-Q Micromechanical Oscillators and Filters for Communications". C. T.-C. Nguyen, IEEE Intl. Symp. Circ. Sys., 2825-2828 (1997). PA0 4) A.-C. Wong, H. Ding, C. T.-C Nguyen, "Micromechanical Mixer+Filter", Tech. Dig. of I.E.E.E./I.E.D.M., San Francisco, Calif., Dec. 198, pp 471-474.
Reverting to the foregoing publications, references (1 through 3) are primarily directed to the general field of utilizing various MEMS devices which are adapted to replacing communication elements. These publications are directed to the description of various conductors, filters and the like which have been constructed using micro-lithography and integrated circuit processing, and essentially are only of limited significance as representing technological background material with respect to the inventive concept.
A solution which is directed to solving the problem of carrier signal mixing and filtering employing micro electro-mechanical system (MEMS) devices concerning the aspects carrier signal mixing and filtering is disclosed in reference 4). This device consisted of two parallel clamped-clamped beam resonators (cantilevers), which have been coupled together with an insulating mechanical bridge. Both resonants are fabricated to possess a natural frequency IF. The input signal (RF) is capacitively coupled to one resonator, which in turn is electrically connected to a sinusoidal local oscillator (LO). The natural vibrational frequency of that resonator is RF-LO. Because of non-linear aspects of the resonator, the resonator mixes the incoming RF signal with the LO signal, and converts it into mechanical motion. This motion is mechanically coupled using the bridge, to the second resonator, which in turn is electrically connected to a DC bias. The mechanical motion induced in this resonator is then capacitively detected as the output signal. The fabricated device operates at a resonant frequency of 27 MHz. Because of the need for an insulating coupling beam between the two conducting resonators, the device is constituted of polysilicon, and then the beams doped using ion implantation. This increased the resistance of the beams considerably over that of metallic components, and consequently the increased insertion losses of the component when used in a circuit. Further, the device is about 20 .mu.m.times.20 .mu.m in size, rendering it difficult to encapsulate for protection against further IC processing.
In addition to the foregoing publications, prior patented devices which relate to electronic mixer-filters are set forth hereinbelow but which fail to provide a system and method utilizing the inventive MEMS device for communication signal mixing and filtering, in a manner analogous to that contemplated by the present invention.
For instance, Fraise U.S. Pat. No. 4,516,271, "Microwave Mixer with Recovery of the Sum Frequency", concerns the use of a wave-guide cavity to mix and filter RF signals. The function of this device is similar to that of the present concept; however, it uses reflection of electro-magnetic waves to process the signal in contrast to the mechanical resonator used here.
Sakamoto U.S. Pat. No. 5,083,139, "Pulse Radar and Components Therefor", also mixes and filter RF signals using the interference of electro-magnetic waves.
Scheinberg U.S. Pat. No. 5,563,545, "Low Cost Monolithic GaAs Upconverter Chip", uses a standard "tank-circuit" consisting of inductors, capacitors and a variable resistor to achieve mixing and filtering of an RF signal.
Kennan U.S. Pat. No. 5,649,312 "MMIC Downconverter for a Direct Broadcast Satellite Low Noise Block Downconverter", also uses standard electronic circuit components for mixing and filtering.
Abe et al., U.S. Pat. No. 5,918,168, "Double Super Tuner", uses a dielectric layer for filteration, and references nine other U.S. patents which use similar techniques.
Finally, Berenz et al., U.S. Pat. No. 5,528,769, "High Electron Mobility Transistor Monolithic Integrated Circuit Receiver", uses a "rat-ring" circuit to generate the mixing of the input RF signal to its local oscillator, This technique is also a method using standard circuit components to accomplish the RF mixer-filter step.
None of the foregoing patents suggest using mechanical vibration for this process, and are therefore not applicable to the inventive concept.